Online Casino Paysafe - Craps Virtual Player

By William I. Orr, WGSAI
48 Campbell Lane, Menlo Park, California 94025
linear amplifier design
The designer of a linear amplifier should be concerned with the proper potentials required to make
the power tube operate in a linear manner. The word
linear implies that the output signal of the amplifier is
an amplified replica of the input signal. There's no
such thing as a perfect linear amplifier, and the
designer's problem is to make the practical amplifier
( i e . , the amplifier that can be built) as linear as
possible.
When a linear amplifier is driven by a complex signal, such as the human voice, nonlinearity results in
intermodulation distortion. This unpleasant form of
distortion creates a broad, raspy signal that throws
annoying "buckshot" into adjacent channels. Proper
design and operation of a linear amplifier reduces this
distortion to a minimum.
amplifier circuit and mode
There's a lot of confusion with regard to the so) called
"grounded-grid" amplifier. Rf power ampli-
fiers are classified according to circuitry and mode of
operation. The two classifications should not be confused with one another. For Amateur service, the
two most popular circuits are the grid-driven circuit
and the cathode-driven circuit. As shown in fig. 1,
the circuits are remarkably similar, the most obvious
difference being the placement of the ground point in
relation to the input and output circuits.
The mode of operation refers to the dynamic operating characteristics of the tube (class AB1, class B,
or class C).Characteristics of the classes are given in
reference material listed at the end of this article. For
linear service, the power tube amplifier is commonly
run in either class AB1 or class B service. Thus, modern equipment may have an intermix of circuitry and
mode - the cathode-driven amplifier may be operated in a class AB1 mode, for example, or the griddriven amplifier may be operated in the class B
mode.
So far, I've not discussed the popular groundedgrid amplifier. This is a sloppy term which usually
refers to a- cathode-driven amplifier, working in the
class B mode. "Grounded grid" implies cathode
drive, but in such a circuit the grid may not necessarily be at dc ground potential, especially with
respect to screen voltage (see fig. 2). Rf ground and
dc ground are not always the same in a linear
amplifier, and most circuit engineers shudder at the
use of the term.
amplifier plate circuit
While this series of articles concerns itself with linear, cathode-driven-amplifier design, the remarks
about the plate circuit apply equally well t o grid-driven amplifiers. It is desirable to operate any linear
amplifier with a very minimum of intermodulation
distortion, with high-plate efficiency, and with high
power gain. The latter is especially important, as it
affords maximum power output with a given amount
of drive power. The class B mode of operation meets
these requirements.
Shown in fig. 3 is a graphical representation of a
class B amplifier, showing the operating cycle of the
tube. This is the portion of the electrical cycle over
which the tube grid is driven positive (approaching
+e) with respect to the cathode (or the cathode
driven negative with respect t o the grid). When the
grid potential is highly negative with respect to the
cathode (approaching -el, the tube is cut off and is
inoperative. In the class B amplifier, the operating
cycle is about one-half the electrical cycle, or approximately 180 degrees. The transfer curve plot shown
indicates that the tube delivers power only over onehalf of the electrical cycle and is cut-off during the
other half of the cycle. Does this mean that the out- . . '
put signal consists of half-sine waves as Shown, a"d'?: . - '
sf&.
.
is therefore highly distorted? Not at all.
The amplifier plate circuit (often called the tank circuit) saves the day, since the energy storage ability
,.
'
(Q) of the circuit balances the energy between the
halves of the cycle, much as the flywheel stores energy during the operating cycles of a gasoline engine.
The plate circuit must, therefore, be designed t o
have sufficient 4, or energy storage, for good operation. A Q value of 12 is commonly used for linear
amplifier service, as it provides ample energy storage
and at the same time provides reasonable reduction
of harmonics generated in the amplifier.
- A
By William I. Orr, WGSAI, 48 Campbell Lane,
Menlo Park, California 94025
CATHODE DRIVEN CIRCUIT
SUPPLY
GRID DRIVEN CIRCUIT
(neutrol~zingommtfledl
SUPPLY
SUPPLY
SUPPLY
SAFETY
RESISTOR
+
M DC GROUND
OC GROUND
fig. 1. A comparison between grid-driven and cathode-driven amplifiers. Rf and dc circuits have been simplified for clarity. I n
both cases, the grid- and plate-current meters are placed i n the ground return circuits t o remove any dangerous voltage f r o m the
meter movement. This, however, places the plate supply above dc ground by virtue of the voltage across the plate meter. I f the
meter coil should open, the negative lead of the supply rises t o the value of the plate voltage. As a safety factor, a wirewound
resistor is usually placed across the plate meter, and often the grid meter. The circuit configuration determines the difference
between cathode- and grid-driven service. The applied voltages determine the mode of operation.
a
A rigorous design of the plate circuit calls for manipulation of the plate voltage and current to determine the operating parameters of the tube. The
results of these tedious calculations can be summed
up in simple formulas that provide the designer with
circuit data in everyday terms.
A network is required that matches the plate load
impedance of the power tube to the characteristic
impedance of the transmission line, while at the
same time maintaining a Qvalue of 12. The popular
pi network can do the job. The plate load impedance
(ZL) for a class B rf amplifier can be closely approximated by:
load impedance (ohms)
-
up until the sloping line denoting a particular Amateur band is intersected. The value of the component
is then read horizontally off the y axis. For example,
the required inductance for a plate load of 1560 ohms
for the 15 meter band is about one microhenry - as
close as the graph can be read. Note that capacitor
C1 is commonly referred to as the tuning capacitor
and C2 the loading capacitor.
The graph for C2 tells us that the pi network cannot cope with impedance transformation values
much greater than 100-to-1 at this value of Q. Note
how the curves bunch together and "fall-off the
graph" at plate impedances much higher than 5000
ohms.
plate uoltage
2 x peak dc plate current (amperes)
As an example, a pi network is to be used to match
a pair of 3-5002 tubes to a %-ohm transmission line.
The tubes operate with 2500 volts plate potential
with a peak dc plate current of 800 mA (0.8 amp) for
a PEP input of 2 kW.
load impedance =
2500
-
2 X 0.8
TETROOE
TUBE
LZ
Thus, the pi network plate circuit has to match a load
impedance of 1560 ohms to a %-ohm termination.
designing the
plate circuit network
The approximate values of the pi network can be
determined from three simple graphs, The plate
inductance from fig. 4, the tuning capacitance (C1)
from fig. 5, and the loading capacitance (c2) from
fig. 6. The graphs are entered at the x axis and read
+
-
SCREEN
SUPPLY
BUS
SUPPLY
+
MI- M
1560ohms
R F GROUND
f
c-
LI
2
;fi
GROUND
'
t
-
fig. 2. Diagram of the so-called "grounded-grid" amplifier.
The grid and screen elements are bypassed t o ground as far
as rf is concerned, but each element has normal operating
voltages applied and are "above ground" as far as dc is concerned. Metering is inserted i n the supply return leads t o dc
ground. Rf ground is placed at the positive screen voltage
level. This eliminates the screen bypass capacitor, a tricky
component that often causes circuit instability a t the higher
frequencies.
A more accurate, computer-derived summary of pi
network values is given in table 1. Note that, for a
given plate impedance, when the operating frequency is doubled the capacitance and inductance values
are halved. (Fifteen- and forty-meter constants are
related by a factor of three as 21 MHz is the third harmonic of 7 MHz.)
coil winding
Winding plate coil L1 to a given value of inductance takes an inductance meter, ora degree of experPLATE
CURRENT
AXIS
a
G
TRANSFER
CURVE
PLATE
SIGNAL
fig. 4. Plot of the plate inductance vs. plate load impedance
for the high frequency Amateur bands (Q = 12).
POINT(E1
TUBE CUTOFF
W R T I O N O F DRIVE
CYCLE -
1
--
--
I I
I / 2 CYCLE
GRID
VOLTAGE
AXIS
+
e- GRID DRIVE VOLTAGE
I * INSTANNNEOUS PLATE CURRENT
E = BIASPOINT
to=
WIESCENT ON
PLATE
TRANSFER
CURRENT
CURVE
I
-.
I
I
I
I
I
DRIVE
SIGNAL
I
1
I
Ce
fig. 3. Transfer curve and operating cycle for a class B amplifier. The transfer curve is determined by a static test of the
tube where plate current is plotted against grid bias. Once
the transfer curve is established, the operating cycle may be
determined. The sine wave drive signal (e) is drawn about
the bias line, determining both the zero-signal plate current
(i,) and the peak plate current (i,,,).
Note that when the
grid driving signal swings negative, no plate current is
drawn and the tube is cut-off for one-half cycle. Pulses of
plate current only appear when the drive signal is positive
w i t h respect t o the bias voltage. Thus, the output waveform
of a class B rf amplifier consists of a series of half-cycles,
much i n the manner of a half-wave rectifier. The distorted
waveform is restored t o a sine wave by the plate tank circuit
which, by virtue of its Q, or flywheel effect, stores energy
on the active half of the cycle and releases it on the inactive
half. Circuit engineers, working from a transfer curve, can
determine actual dc operating potentials f o r a linear
amplifier.
ple slide rule providing direct read-out of the coil
dimensions if the inductance is known. It takes the
hard work out of designing coils.
Once the plate circuit has been designed and built,
it is a good idea to "breadboard" it up and check it
out with a dip-meter before the connections are finally soldered. Coil taps may have to be moved a bit to
compensate for capacitance of the components to
the chassis and adjacent parts.
amplifier-cathode circuit
The cathode-input circuit provides an impedance
match between the 50-ohm coaxial output circuit of
the driverlexciter and the input impedance of the
cathode-driven amplifier (see table 2). The input im-
tise and a dip-meter. A simple formula for calculating
inductance when the coil dimensions are known is:
Inductance
(fl)=
R2N2
gR + IOS
where R is the radius of the coil in inches
S is the length of the coil winding in inches
N is the number of turns
These calculations have been simplified in the
ARRL type-A "Lightning Calculator," which is a sim-
36
july 1979
fig. 5. plot of the tuning capacitance (C1) vs. plate
load impedance ( Q = 12).
pedance (Zt) of a cathode-driven tube is related to
the ratio of the peak cathode signal voltage to the
peak cathode current (sum of grid and plate currents), and is commonly given in the tube data sheet.
For the 3-5002 at 2500 volts, it is about 110 ohms.
And for two tubes in parallel, it is about 55 ohms, but
only over the operating cycle.
It is tempting to jump to the conclusion that if the
amplifier input impedance is about 55 ohms and the
coaxial line impedance driving it is 50 ohms, that no
cathode impedance matching circuit is required. In
fact, many commercially manufactured amplifiers
leave it out for economy's sake. This omission is poor
engineering practice, as the circuit Q is required in
the cathode circuit as well as in the plate circuit.
Omission of the cathode-tuned circuit can lead to
distortion of the driving signal, increased intermodulation distortion, reduced amplifier efficiency, and
driver loading problems. A circuit Qof 2 is adequate,
and a simple rule of thumb is that the network circuit
capacitances at resonance should be about 20 pF per
meter of wavelength for one-to-one impedance
transformation.
practical amplifier circuit
Armed with the information discussed so far, it is
possible t o draw up a schematic for a cathode driven,
2-kW PEP linear amplifier using two 3-5002 tubes in
parallel (see fig. 7 ) . This is a true "grounded-grid"
circuit, as the grids are at both dc and rf ground
potential.
fig. 6. Plot of the loading capacitance (C2) vs. plate
load impedance (Q = 121.
Note that plate and grid currents are measured in
the cathode return circuit. This requires the amplifier
plate power supply to "float" a little above ground
potential in order to insert a meter in the negative
lead to measure plate current. This removes the
lethal plate voltage from the meter. The grid meter is
out of the critical rf ground return path, which simplifies the metering circuit. A filament voltmeter is
included. Filament voltage should be held to within
LOAD = 5 0 OHM
table 1. Computer-derived values for a pi network having a Q o f 12 and working into a 50ohm load. Values for C1 include the output capacitance of the tubes. These values are taken
from a computer program derived by Bob Sutherland, W6PO.
ZL plate load impedance (ohms)
component
band
160
80
C1
40
20
15
10
1000
1060
546
273
136
91
68
1500
690
364
182
91
61
45
2000
531
273
136
68
45
34
2500
430
220
110
55
37
30
3000
354
182
91
45
30
23
3500
309
159
80
40
26
20
4000
265
136
68
34
23
17
5000
212
109
55
27
18
14
f 5 per cent of 5 volts, and it is prudent to monitor
this voltage when expensive tubes are used. A plate
voltmeter may be included in the amplifier, but it is
easier t o place it in the power supply.
Amplifier standby plate current is reduced by
means of a 10-kilohm, 25-watt cathode resistor
which is shorted out by the VOX relay of the exciter,
causing the tubes to operate at the proper resting
plate current when the amplifier is on the air. A zener
diode is placed in series with the cathode dc return
path t o reduce the quiescent plate current during
amplifier operation.
A %-ohm wirewound resistor from the negative
side of the plate supply to ground makes certain that
the negative supply terminal does not rise to the
value of the plate voltage if the positive side of the
supply is accidentally shorted to ground.
Two reverse-connected diodes are shunted across
the safety resistor to limit any transient surges under
a shorted condition which might cause wiring insula-
tion breakdown. In addition, the diodes protect the
meters from transient currents. A resistor across the
zener diode provides a constant load for it and prevents cathode voltage from soaring if the zener safety fuse opens.
Note that a 10-ohm, %-watt wirewound resistor is
placed in series with the B-plus lead to the plate rf
choke. This resistor serves as a vhf choke to suppress harmonic currents in the power lead and also
protects the tube and associated circuitry in case of a
flash-over in the tube or plate circuit. The trernendous amount of energy stored in the power supply is
instantaneously "dumped" into the amplifier when a
FOR 2," - 5 0 OHM
0.2
table 2. The pi-network circuit for a cathode-driven amplifier. This chart provides approximate values for the components of the
cathode circuit. Capacitors should be I - k V silver mica or equivalent. The inductor can be wound on a slug-tuned form. Value of
C2 should take into account the cathode-grid capacitance of the tube which appears in parallel with C2 (information is from a
computer program by W6POl.
cathode
z,(m
20
38
july 1979
band
Cl(pFI
C2(pF)
L(pH1
160
80
40
20
15
10
3300
1700
900
440
300
220
4100
2120
1120
560
370
275
2.50
1.34
0.68
0.33
0.22
0.16
cathode
Z,(Q)
75
band
Cl(pF)
C2(pF)
160
80
40
20
15
10
3300
1700
900
440
300
220
2870
1540
770
380
250
180
L~(PHI
3.81
2.05
1.03
0.51
0.34
0.25
C6
0
1
SKY
I / - -
LP
L3
-
J2
OUTPUT
RFC 3
'OM
40M
15M
0 01
001
001
001
d
DANGER- HIGH VOLTAGE
L - _ -
1
- - - ----------
2
3
4
5
6
7
J
8
fig. 7 . Schematic diagram of t h e 3-5002 linear amplifier.
C3
C4
C5
C6
250 pF, 4.5 kV plate spacing - Johnson 154-16
500 pF, 4.5 kV
1000 pF, 500 volt plate spacing
0.001 ,F, 5 kV - Centralab 858s-100
C7, C8 500 pF, 10 kV TV-type "door knob"
C9-C14 0.01 ,F, 500 volt mica capacitor. Ceramic disc is a
suitable substitute if rated 1 kV.
PC 1
Three 100-ohm, 2-watt resistors in parallel
PC 2
Three turns of no. 14 AWG (1.6 m m ) wound with 12.5m m (0.5-inch) diameter and 19-mm (0.75-inch) length
connected in parallel with the resistors. The coil may be
wound around one of the resistors.
flash-over occurs, and much of this destructive energy is dissipated in the resistor.
Many modern-generation Amateurs have never
worked with equipment operating at voltages higher
than 12 volts. This amplifier, with the high-voltage
plate supply, is positively lethal and the operator can
be killed if his hands are inside the unit when the high
voltage is on. It is imperative, therefore, that safety
switches be incorporated in the amplifier design. It is
poor engineering practice to leave these devices out!
S4 isa normally open, pushbuttondevice that isclosed
only when the lid is placed on the amplifier enclosure. S3 is a shorting switch that shorts the high voltage to ground when the lid is removed. Construction
of this special switch will be covered in a future article. Always remember - high voltage kills! Take
necessary precautions.
RFC 1
RFC 2
RFC 3
TI
Blower
50 pH; 14 bifilar turns of no. 10 AWG (2.6 rnm) enameled
wire wound on ferrite core 12.5 cm (5 inches) long and
12.5 c m (0.5 inch) in diameter (Indiana General CF-503 or
equivalent).
100 pH, 1 ampere dc; 112 turns no. 26 AWG (0.4 mm)
spacewound wire diameter on 2.5 c m (1 inch) ceramic
f o r m 15 c m ( 6 inches) long (Centralab X-3022H
insulator). Series resonant at 24.5 MHz with terminals
shorted ( B 8 W 800).
2.5 mH, 100 mA
5 volts at 30 amps (Chicago-Standard P-4648)
13 cu. ft./min. Use a no. 3 impeller at 3100 rpm (Ripley
8472, Dayton 1C-180, or Redmond AK-2H-OlAX)
Although not shown on the schematic, it is a good
idea to use a filament transformer having a primary
winding tapped for 105, 115, and 125 volts. This provides a plus or minus ten per cent adjustment from a
normal line voltage of 115 volts. If a closer filament
adjustment is desirable, the transformer can be run
on the 105 volt tap with a rheostat in series with the
primary winding to place the filament voltage "on the
nose."
The plus and minus leads to the high voltage supply should be run through high-voltage connectors
and high-voltage cable. Test prod wire having a 10kV breakdown is satisfactory. As an alternative, RG58lU coaxial cable can be used for high-voltage leads
along with PL-259 plugs and reducers and SO-239
receptacles. The shield of the coaxial line is grounded
by the connectors.
ham radio